1. Field of the Invention
This invention relates to a pnuematic percussion tool such as a nailing machine having a head valve device for controlling the supply and discharge of compressed air which is utilized for reciprocally moving a percussion piston for driving a nail etc. into a work.
2. Description of the Prior Art
Japanese Patent Publication No. 57-57234 discloses a pneumatic nailing machine including a body, a cylinder mounted within the body, a percussion piston connected with a driver for percussion operation and reciprocally movable within the cylinder, a percussion piston chamber formed in the body adjacent one of the stroke ends of the percussion piston and communicating with the cylinder, a main air chamber formed in the body adjacent the percussion piston chamber for storing compressed air supplied from a compressed air source, and a head valve device mounted on the body for permitting or preventing communication between the percussion piston chamber and the main air chamber and for permitting or preventing communication betwen the percussion piston chamber and the outside, a head valve chamber formed in the body opposed to the percussion piston chamber, a head valve disposed between the percussion piston chamber and the head valve chamber and movable in response to the pressure difference therebetween, an outlet port formed in the body for communication between the percussion piston chamber and the outside through the head valve, a passage formed in the body for supllying the compressed air from the air source to the head valve chamber, and a trigger valve connected with the end of the passage for selectively connecting the same with the air source or the outside.
The trigger valve is operable by a trigger and the trigger valve is normally held in a position to connect the passage with the air source, so that the compressed air is supplied to the head valve chamber for keeping the head valve in a first position to prevent communication between the main air chamber and the percussion piston chamber and to permit communication between the percussion piston chamber and the outside. In this stage, the percussion piston is positioned at the stroke end adjacent the percussion piston chamber (the upper stroke end).
When the trigger is pulled to connect the passage with the outside, the head valve is moved by the pressure difference between the main air chamber and the head valve chamber from the first position to a second position in which the head valve prevents communication between the percussion piston chamber and the outside while permitting communication between the main air chamber and the outside of the body. The percussion piston is therefore moved toward another stroke end so as to perform the nailing operation.
The trigger is therafter released to connect the passage with the compressed air source, so that the head valve is moved to return the first position and the percussion piston returns to the upper stroke end.
In the nailing operation of this prior art, the compressed air stored in the head valve chamber is exhausted only through the passage connecting the trigger valve. Therefore, the compressed air cannot be smoothly and rapidly exhausted from the head valve chamber. This may cause delay of movement of the head valve for supplying the air from the main air chamber to the percussion piston chamber, so that the nailing force of the percussion piston is lowered. This may further cause unreliable movement of the percussion piston, so that the nailing machine cannot be used for momentary nailing operation.
Japanese Patent Publication No. 58-48310 discloses a head valve device including a first and a second head valve. A passage connecting a trigger valve with a head valve chamber is connected with the head valve chamber through a first and second channel formed in a body. A third channel is formed in the body for supplying air from a main air chamber to a gap between the first and second head valves. When a trigger is operated to change the trigger valve for opening the passage into the outside, the air in the head valve chamber is exhausted through the passage via the first and second channels while the air supplied to the gap between the first and seocnd chamber moves the first head valve upwardly away from the first head valve. When the first head valve reaches its uppermost position to close the first channel, the air supplied into the gap flows into the second channel, so that the second head valve is moved upwardly for communication between the main air chamber and a percussion piston chamber by the pressure difference between the gap and the main air chamber. The third channel is closed by the second head valve when the second head valve reaches its uppermost position.
Thus, in this prior art, the air pressure in the main air chamber is used to positively move the first head valve. The communication between the main air chamber and the percussion piston chamber is, however, made by the movement of the second head valve baded on the pressure difference between the gap and the main air chamber.
The air flown into the gap from the main air chamber is exhausted to the outisde through the second channel and the passage when the first head valve reaches its upper most position. Therefore, leakage of the air is caused in the main air chamber. Further, the second head valve cannot be moved rapidly since the pressure difference between the gap and the main air chamber is relatively small because of the presence of the flow of air from the main air chamebr to the gap at the first stage.
Japanese Utility Model Publication No. 58-45025 discloses a head valve device having a head valve which includes a communication hole in a radial direction for communiction between a percussion piston chamber and an outlet port formed in a body. An annular recess is formed on the inner surface of the head valve chamber. The annular recess permits communication between the head valve chamber and the outlet port through the communication hole when the seal member mounted on the head valve moves into the annular recess. Thus, the compressed air stored in the head valve chamber may be exhausted through the outlet port based on the movement of the head valve passage conneting the head valve chamber and a trigger valve as well as the outlet port when the trigger has been changed to the outside.
However, the compressed air supplied into the head valve for returning the head valve for preventing communication between the main air chamber and the percussion piston chamber may also escape from the outlet port when the seal member has moved into the annular recess. Therefore, this may cause leakage of the air.